Microbial colonization influences early B-lineage development in the gut lamina propria

Primary B-cell development is thought to be restricted to the bone marrow; here it is shown to occur also in intestinal tissues of postnatal mice, that it peaks at the time of weaning and is increased upon colonization of germ-free mice, and is thus influenced by commensal microbes. Second site for B-cell formation Primary B-cell development is thought to be restricted to the bone marrow, but here Frederick Alt and colleagues present the surprising finding that it also occurs in the gut, where it is stimulated by the gut microbes. The authors describe a population of early B-lineage cells developing within the intestinal mucosa — specifically in the lamina propria — of postnatal mice. B-cell production peaks at the time of weaning and is increased upon colonization of germ-free mice. The repertoire of these B cells differs from that of cells derived from the bone marrow, and may be shaped by commensal microbes. The RAG1/RAG2 endonuclease (RAG) initiates the V(D)J recombination reaction that assembles immunoglobulin heavy ( IgH ) and light ( IgL ) chain variable region exons from germline gene segments to generate primary antibody repertoires 1 . IgH V(D)J assembly occurs in progenitor (pro-) B cells followed by that of IgL in precursor (pre-) B cells. Expression of IgH μ and IgL (Igκ or Igλ) chains generates IgM, which is expressed on immature B cells as the B-cell antigen-binding receptor (BCR). Rag expression can continue in immature B cells 2 , allowing continued Igκ V(D)J recombination that replaces the initial VκJκ exon with one that generates a new specificity 3 , 4 , 5 . This ‘receptor editing’ process, which can also lead to Igλ V(D)J recombination and expression 3 , 6 , 7 , provides a mechanism whereby antigen encounter at the Rag -expressing immature B-cell stage helps shape pre-immune BCR repertoires. As the major site of postnatal B-cell development, the bone marrow is the principal location of primary immunoglobulin repertoire diversification in mice. Here we report that early B-cell development also occurs within the mouse intestinal lamina propria (LP), where the associated V(D)J recombination/receptor editing processes modulate primary LP immunoglobulin repertoires. At weanling age in normally housed mice, the LP contains a population of Rag -expressing B-lineage cells that harbour intermediates indicative of ongoing V(D)J recombination and which contain cells with pro-B, pre-B and editing phenotypes. Consistent with LP-specific receptor ed